This is related to issue 769426. But encounter a different error in Android layer.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b8b90143fe7723118041beecd79d813923c509e7

commit b8b90143fe7723118041beecd79d813923c509e7
Author: Xing Liu <xingliu@chromium.org>
Date: Thu Oct 05 23:26:36 2017

Download Service: Handle transient download in target determiner.

This CL fixed an issue that transient download may trigger android UI
and cancel the download in download resumption.


How it happens: In resumption, we feed current path as initial path to
target determiner instead of forced path. Target determiner may change
the download path from DS internal storage to external storage, this
creates the following issues:
1. External storage needs permission.
2. Conflict files are not rewritten, but needs user confirmation with
a snackbar, because we don't have webcontents, target determiner will
delete itself and fail the download with reason USER_CANCEL.

All the Android leak will call into
DownloadManagerService::OnDownloadCanceled, where it shows two kinds of
error messages in toast.

This issue may be also related to issue 762173.

Bug:  769805 , 769426
Change-Id: I1ac6d2446fcc05c00517a65b465d7bf1ea2ccfeb
Reviewed-on: https://chromium-review.googlesource.com/693400
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506908}
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/android/download/download_manager_service.cc
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/chrome_download_manager_delegate.cc
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/download_path_reservation_tracker.h
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/download_stats.cc
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/download_stats.h
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/download_target_determiner.cc
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/chrome/browser/download/download_target_determiner_unittest.cc
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/content/public/browser/download_item.h
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/content/public/browser/download_url_parameters.h
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/b8b90143fe7723118041beecd79d813923c509e7/tools/metrics/histograms/histograms.xml